The top U.S. diplomat in Seoul pressed China, Wednesday, to use its leverage over North Korea to persuade it to curb its provocative behavior, as tension remains high over the North’s nuclear program and the shelling last month of Yeonpyeong Island.

“It is critically important that China continue to play a strong role in making clear to North Korea that there are consequences for its actions,” U.S. Ambassador to Seoul Kathleen Stephens said in a speech to the U.S. Chamber of Commerce.

“We hope China will work with us to send a clear, unmistakable message to North Korea that they have to demonstrate a ‘seriousness of purpose’ and end their provocative actions.”

China is seen to wield the most influence over the North as it is its main benefactor and last major ally.

In the wake of the Nov. 23 artillery attack that killed four South Koreans, China has called for an emergency meeting of the six-party talks on the North’s denuclearization, a proposal Beijing says the North has agreed to.

But Washington, Seoul and Tokyo have thus far balked at the meeting, saying such a move would be rewarding the North’s provocative behavior.

Stephens offered her condolences for the lost lives and expressed her government’s steadfast support.

“The United States stands firmly with the Republic of Korea. In the aftermath, we have been working very closely through military and diplomatic channels to develop countermeasures to respond firmly to further provocations from the North and chart a diplomatic path forward,” she said.

The remarks came as Deputy U.S. Secretary of State James Steinberg led a delegation to China to urge Beijing to exert its leverage over the North.

Concern over the North’s nuclear program spiked again Tuesday when the U.S. State Department said the North has at least one other uranium enrichment plant in addition to the one it revealed to U.S. experts last month.

Pyongyang says the new program is intended for civilian use, but it is feared it can be converted to create nuclear weapons.

The newly-revised free trade agreement between Korea and the United States (KORUS FTA), once ratified by both parliaments, will add a measure of stability to the region as well as generate economic benefits for both sides, Stephens said.

The pact “will send a strong signal throughout the region and the world of U.S. commitment and relevance in the Asia-Pacific region” and reach a “new level with new benefits of shared prosperity and security not only for our citizens but for the Asia-Pacific region,” she said.

The two sides settled on a final agreement earlier this month, forging supplemental amendments dealing with automobile trade.

Under the revised pact, South Korea agreed to soften its automotive safety and environmental standards, but kept the thorny domestic issue of U.S. beef off the table.

Responding to an audience member who insisted that the division of Korea should be blamed on the United States and Russia for their roles after World War II, Stephens, once a Peace Corps member in Korea, said the division was “one of the great tragedies of the 20th century.”

“The division of Korea happened through no fault of the Korean people,” she said. “It is the policy of my government and my own deepest personal desire to see the peaceful and democratic reconciliation and unification of Korea on terms that are acceptable to the Korean people. That is what we are working for.”